NOTES ON KERNEL OSS-EMULATION

Modules

ALSA provides a
powerful OSS emulation on the kernel.
The OSS emulation for PCM, mixer and sequencer devices is implemented
as add-on kernel modules, snd-pcm-oss,
snd-mixer-oss and snd-seq-oss.
When you need to access the OSS PCM, mixer or sequencer devices,
the corresponding module has to be loaded.

For loading these modules automatically, define the aliases in
/etc/modules.conf like below:

Here you don't need to define service-1-1 and
service-1-8 because there is only one sequencer
device. Similarly, you can add definitions for the third or
later cards as sound-service-X-Y.

The OSS-MIDI is emulated directly in the ALSA rawmidi module,
therefore no extra module exists for that purpose.

The currently available OSS configuration is shown in
/proc/asound/oss/sndstat. This shows in the same
syntax of /dev/sndstat, which is available on the
commercial OSS driver. On ALSA, you can symlink
/dev/sndstat to this proc file.

Please note that the devices listed in this proc file appear only
after the corresponding OSS-emulation module is loaded. Don't worry
even if "NOT ENABLED IN CONFIG" is shown in it.

Device Mapping

ALSA supports the following OSS device files:

PCM

/dev/dspX

/dev/adspX

Mixer

/dev/mixerX

MIDI

/dev/midi0X

/dev/amidi0X

Sequencer

/dev/sequencer

/dev/sequencer2 (aka
/dev/music)

where X is the card number from 0 to 7.

NOTE: Some distributions have the device files like
/dev/midi0 and /dev/midi1. They are
NOT for OSS but for tclmidi, which is
a totally different thing.

Unlike the real OSS, ALSA cannot use the device files more than the
assigned ones. For example, the first card cannot use
/dev/dsp1 or /dev/dsp2, but only
/dev/dsp0 and /dev/adsp0.

As seen above, PCM and MIDI may have two devices. Usually, the first
PCM device (hw:0,0 in ALSA) is mapped to
/dev/dsp0 and the secondary device
(hw:0,1) to /dev/adsp0 (if available).
For MIDI, /dev/midi0 and /dev/amidi0
for the first and second devices, respectively.

You can change this device mapping via the module options of
snd-pcm-oss and snd-rawmidi. In the
case of PCM, the following options are available for
snd-pcm-oss:

dsp_map

PCM device number assigned to /dev/dspX
(default = 0)

adsp_map

PCM device number assigned to /dev/adspX
(default = 1)

For example, to map the third PCM device (hw:0,2)
to /dev/adsp0, define like this:

options snd-pcm-oss adsp_map=2

The options take arrays. For configuring the second card,
specify two entries separated by comma. For example, to map the
third PCM device on the second card to /dev/adsp1,
define like below:

options snd-pcm-oss adsp_map=0,2

To change the mapping of MIDI devices, the following options are
available for snd-rawmidi:

midi_map

MIDI device number assigned to /dev/midi0X
(default = 0)

amidi_map

MIDI device number assigned to /dev/amidi0X
(default = 1)

For example, to assign the third MIDI device on the first card to
/dev/midi00, define as follows:

options snd-rawmidi midi_map=2

PCM Mode

As default, ALSA emulates the OSS PCM with so-called plugin layer,
i.e. tries to convert the sample format, rate or channels
automatically when the card doesn't support it natively.
This will lead to some problems for some applications like quake or
wine, especially if they use the card only in the MMAP mode.

In such a case, you can change the behavior of PCM per application by
writing a command to the proc file. There is a proc file for each PCM
stream,
/proc/asound/cardX/pcmY[cp]/oss,
where X is the card number (zero-based), Y the PCM
device number (zero-based), and p is for playback
and c for capture, respectively. Note that this
proc file exists only after snd-pcm-oss module is
loaded.

The command sequence has the following syntax:

app_name fragments fragment_size [options]

app_name is the name of application with (higher
priority) or without path.
fragments specifies the number of fragments or zero
if no specific number is given.
fragment_size is the size of fragment in bytes or
zero if not given.

options is the optional parameters. The following
options are available:

disable

The application tries to open a pcm device for
this channel but does not want to use it.

direct

Don't use plugins

block

Force block open mode

non-block

Force non-block open mode

The disable option is useful when one stream direction (playback or
capture) is not handled correctly by the application although the
hardware itself does support both directions.
The direct option is used, as mentioned above, to bypass the automatic
conversion and useful for MMAP-applications.
For example, to playback the first PCM device without plugins for
quake, send a command via echo like the following:

% echo "quake 0 0 direct" > /proc/asound/card0/pcm0p/oss

While quake wants only playback, you may append the second command
to notify driver that only this direction is about to be allocated:

% echo "quake 0 0 disable" > /proc/asound/card0/pcm0c/oss

The permission of proc files depend on the module options of snd.
As default it's set as root, so you'll likely need to be superuser for
sending the command above.

The block and non-block options are used to change the behavior of
opening the device file.
As default, ALSA behaves as defined in POSIX, i.e. blocks the file
when it's busy until the device becomes free (unless
O_NONBLOCK is specified). Some applications assume
the non-block open behavior, which are actually implemented in
some real OSS drivers.

This blocking behavior can be changed globally via
nonblock_open module option of
snd-pcm-oss. For using the non-block mode as
default for OSS devices, define like the following:

options snd-pcm-oss nonblock_open=1

You can check the currently defined configuration by reading the proc
file. The read image can be sent to the proc file again, hence you
can save the current configuration

% cat /proc/asound/card0/pcm0p/oss > /somewhere/oss-cfg

and restore it like

% cat /somewhere/oss-cfg > /proc/asound/card0/pcm0p/oss

Also, for clearing all the current configuration, send
erase command as below:

% echo "erase" > /proc/asound/card0/pcm0p/oss

Mixer Elements

Since ALSA has completely different mixer interface, the emulation of
OSS mixer is relatively complicated. ALSA builds up a mixer element
from several different ALSA (mixer) controls based on the name
string. For example, the volume element
SOUND_MIXER_PCM is composed from "PCM Playback
Volume" and "PCM Playback Switch" controls for the playback
direction and from "PCM Capture Volume" and "PCM Capture Switch"
for the capture directory (if exists). When the PCM volume of
OSS is changed, all the volume and switch controls above are
adjusted automatically.

The second column is the base-string of the corresponding ALSA
control. In fact, the controls with "XXX [Playback|Capture]
[Volume|Switch]" will be checked in addition.

The current assignment of these mixer elements is listed in the proc
file, /proc/asound/card>X/oss_mixer,
which will be like the following

VOLUME "Master" 0
BASS "" 0
TREBLE "" 0
SYNTH "" 0
PCM "PCM" 0
...

where the first column is the OSS volume element, the second column
the base-string of the corresponding ALSA control, and the third the
control index. When the string is empty, it means that the
corresponding OSS control is not available.

For changing the assignment, you can write the configuration to this
proc file. For example, to map "Wave Playback" to the PCM volume,
send the command like the following:

% echo 'VOLUME "Wave Playback" 0' > /proc/asound/card0/oss_mixer

The command is exactly as same as listed in the proc file. You can
change one or more elements, one volume per line. In the last
example, both "Wave Playback Volume" and "Wave Playback Switch" will
be affected when PCM volume is changed.

Like the case of PCM proc file, the permission of proc files depend on
the module options of snd. you'll likely need to be superuser for
sending the command above.

As well as in the case of PCM proc file, you can save and restore the
current mixer configuration by reading and writing the whole file
image.

Duplex Streams

Note that when attempting to use a single device file for playback
and capture, the OSS API provides no way to set the format, sample
rate or number of channels different in each direction. Thus

io_handle = open("device", O_RDWR)

will only function correctly if the values are the same in each
direction.

Unsupported Features

MMAP on ICE1712 driver

ICE1712 supports only the unconventional format, interleaved
10-channels 24bit (packed in 32bit) format. Therefore you cannot mmap
the buffer as the conventional (mono or 2-channels, 8 or 16bit) format
on OSS.

USB devices

Some USB devices support only 24bit format packed in 3bytes. This
format is not supported by OSS and no conversion is provided by kernel
OSS emulation. You can use the user-space OSS emulation via libaoss
instead.